The present invention relates generally to an electrostatographic printing apparatus and more particularly to a fusing system for fixing toner material to support substrate. In particular the present invention relates to a release agent donor roller for a toner fixing station in such apparatus.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. The residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support or receiver, such as a sheet of plain paper with subsequent affixing of the image thereto.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the receiver.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the receiver with the unfused toner images thereon between a pair of opposed roller members at least one of which is heated. During operation of a fusing system of this type, the receiver to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roller thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roller systems wherein the fusing roller is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. The silicone rubbers which have been used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e., room temperature vulcanization silicone rubber referred hereinafter referred to as RTV silicone rubber, liquid silicone rubber to as LSR rubber, and high temperature vulcanization type silicone rubber referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the receiver will be retained by the heated fuser roller and not penetrate into the receiver surface. This tackified material will stick to the surface of the fusing roller and come in contact with the subsequent receiver sheet bearing a toner image to be fused. A tackified image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tackified image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused there between, toner may be transferred from the fuser roller to the pressure roll. These conditions are referred to in the copying art as "offset". Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the adhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It has also been proposed to provide toner release agents such as silicone oil, in particular, poly(dimethylsiloxane), which is applied on the fuser roller to a thickness of the order of about 1 micron to act as a polymeric release agent. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roller environment. In practice, a thin layer of poly(dimethylsiloxane) (silicone oil) release agent is applied to the surface of the heated roller to form an interface between the roller surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roller surface.
Some recent developments in fuser rollers, polymeric release agents and fusing systems are described in U.S. Pat. No. 4,264,181 to Lentz et al., U.S. Pat. No. 4,257,699 to Lentz and U.S. Pat. No. 4,272,179 to Seanor. These patents describe fuser rollers and methods of fusing thermoplastic resin toner images to a substrate wherein a polymeric release agent having functional groups is applied to the surface of the fuser roller. The fuser roller comprises a base member having an elastomeric surface with a metal containing filler therein which has been cured with a nucleophilic addition curing agent. Exemplary of such fuser roller is an aluminum base member with a poly(vinylidenefluoride-hexafluoropropylene) copolymer cured with bisphenol curing agent having lead oxide filler dispersed therein and utilizing a mercapto functional polyorganosiloxane oil as a polymeric release agent. In those fusing processes, the polymeric release agents have functional groups (also designated as chemically reactive functional groups) which interact with the metal containing filler dispersed in the elastomer or resinous material of the fuser roller surface to form a thermally stable film which releases thermoplastic resin toner and which prevents the thermoplastic resin toner from contacting the elastomer material itself. The metal oxide, metal salt, metal alloy or other suitable metal compound filler dispersed in the elastomer or resin upon the fuser roller surface interacts with the functional groups of the polymeric release agent. Preferably, the metal containing filler materials do not cause degradation of or have any adverse effect upon the polymeric release agent having functional groups. Because of this reaction between the elastomer having a metal containing filler and the polymeric release agent having functional groups, excellent release and the production of high quality copies are obtained even at high rates of speed of electrostatographic reproducing machines.
While the mechanism involved is not completely understood, it has been observed that when certain polymeric fluids having functional groups are applied to the surface of a fusing roller having an elastomer surface with a metal oxide, metal salt, metal, metal alloy or other suitable metal compounds dispersed therein there is an interaction (a chemical reaction, coordination complex, hydrogen bonding or other mechanism) between the metal of the filler in the elastomer and the polymeric fluid having functional groups so that the polymeric release agent having functional groups in the form of a liquid or fluid provides an excellent surface for release which having an excellent propensity to remain upon the surface of the fuser roller. Regardless of the mechanism, there appears to be the formation of a film upon the elastomer surface which differs from the composition of the elastomer and the composition of the polymeric release agent having functional groups. This film, however, has a greater affinity for the elastomer containing a metal compound than the toner and thereby provides an excellent release coating upon the elastomer surface. The release coating has a cohesive force which is less than the adhesive forces between heated toner and the substrate to which it is applied and the cohesive forces of the toner. The interaction between the functional group of the polymeric release agent and the metal of the elastomer containing metal leads to an overall diminution of the critical or high surface energy of the metal in the metal containing filler. The reaction of a functional group of the polymeric release agent is especially useful for nonsilicone elastomer based fusing systems; however, advantages can also be seen in the use of funtionalized polymeric release agent with silicone elastomer based fusing rollers in offset reduction.
U.S. Pat. Nos. 4,029,827, to Imperial et al., U.S. Pat. No. 4,101,686 to Strella et al. U.S. Pat. No. 4,185,140 also to Strella et al also disclose the use of polymeric release agents having functional groups which interact with the fuser roller to form a thermally stable renewable self cleaning layer having superior release properties for electroscopic thermoplastic resin toners. In particular, U.S. Pat. No. 4,029,827 is directed to the use of polyorganosiloxane having mercapto functionality as polymeric release agents. U.S. Pat. Nos. 4,101,686 and 4,185,140 are directed to polymeric release agents having functional groups such as carboxy, hydroxy, epoxy, amino, isocyanate, thioether and mercapto groups as release fluids.
According to prior art techniques the toner release agents may be applied to the fuser roller by several delivery mechanisms including wicking, impregnating webs and by way of a release agent donor roller which may comprise an EPDM (terpolymer elastomer made from ethylene, propylene and diene monomer) core with a thin sleeve of Teflon, PFA (E.I. DuPont De Nemours) which is an independent extruded thin sleeve of material which is bonded onto the core.
The use of such a sleeve is very expensive and the manufacturing of such a donor roll, is tedious and inefficient, the yield being relatively low since so many of the sleeves are damaged during manufacture. Furthermore, in a fusing assembly such as that illustrated in FIG. 1, which will be described in greater detail hereinafter, such a sleeved release agent donor roller is ineffective in that since the release agent donor roller is driven by frictional engagement with the fuser roll, the hard Teflon PFA coating has a relatively low coefficient of friction difficulties are presented in providing the necessary driving component.
Another technique has been with the use of a release agent donor roller made of a high temperature vulcanized silicone rubber material. Another development is described in U.S. Pat. No. 4,659,621 to Finn et al. wherein a release agent donor roller is described as having a conformable donor surface comprising the crosslinked product of at least one addition curable vinyl terminated or vinyl pendant polyorganosiloxane, a polyfunctional silicone hydride crosslinking agent crosslinking catalyst and finely divided filler. While these silicone elastomer donor rolls have been commercially successful in some commercial applications they suffer from certain difficulties in that they tend to swell by being in contact with a silicone oil release agent which migrates or is absorbed into the silicone rubber. While a small degree of swelling may be acceptable if it is uniform, failure of such rolls has been observed by excessive swelling over a period of operation wherein the release agent donor roller may actually be twice the original size. Under such circumstances, the silicone rubber release agent donor roller may no longer function in providing a uniform layer of release fluid to the fuser roll. Further, while donor rolls described in U.S. Pat. No. 4,659,621 have attractive oil delivery capabilities in that they are capable of transporting sufficient quantities of functional polymeric release agent to the fuser roller to form the interfacial barrier layer between the fuser roller and the toner they also tend to swell with the polymeric release agent penetrating the rubber whereby there may be an interchange of the poly(dimethylsiloxane) release agent with the poly(dimethylsiloxane) in the silicone rubber network leading to breakdown of the network and a lower crosslinked network thereby reducing the toughness of the silicone rubber barrier layer as more polymeric release agent penetrates the surface. This difficulty is particularly pronounced when operating at temperatures in excess of 300.degree. F. and at very high printing speeds of the order of 135 copies per minute. Failure is observed by a mechanism referred to as chunking wherein pieces actually separate from the surface of the roller because the rubber has such low toughness. These small pieces can actually show up on a copier print. As a result of this chunking process the delivery of polymeric release agent is no longer uniform to the fuser roll. An additional failure mode is referred to as debonding wherein the swelling of the silicone rubber has become so significant that it actually delaminates from the core of the donor roll. A similar situation has been described to occur to poly(dimethylsiloxane) based fuser rollers in U.S. Pat. No. 4,777,087 by Heeks et al.
Another recent development is described in U.S. Pat. No. 5,061,965 to Ferguson et al. This describes the use of a release agent donor roller made of a base roller, an intermediate comfortable silicone elastomer layer, and an elastomer release layer comprising poly(vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene) where the vinylidene fluoride is present in an amount &lt;40 mole %, a metal oxide present in an amount sufficient to interact with polymeric release agent having functional groups to transport a sufficient amount of polymeric release agent to provide an interfacial barrier layer between the fusing surface and the toner. This release agent donor roller suffers from the polymeric release agent wetting capability between the nonfunctional PDMS release agent and the nonreactive release agent donor roller surface since the invention counts on the polymeric release agent having functional groups to react with the metal oxide which is dispersed in the fluoroelastomer layer.
A more recent development described in U.S. Pat. Nos. 5,141,788 and 5,166,031 to S. Badesha wherein a release agent donor roller comprising a supporting substrate having an outer layer of a surface grafted or volume grafted polyorganosiloxane formed by dehydrofluorination of said fluoroelastomer by nucleophilic dehydrofluorinating agent, followed by addition polymerization by the addition of an alkene functionalized polyorganosiloxane and a polymerization initiator. Fabricated release agent donor roller used for supplying conventional silicone oil release agent showing 4.3 million copies without failure. Although these rolls provide long life, non-oil swelling, and can be used with non-functional PDMS release agent, the manufacturing of such a release agent donor roller is tedious, inefficient, and expensive.